Specific $Na+$ and $K+$ cation effects on the interfacial water molecules at the air/aqueous salt solution interfaces probed with nonresonant second harmonic generation

Here we report on the polarization dependent nonresonant second harmonic generation (SHG) measurement of the interfacial water molecules at the aqueous solution of the following salts: NaF, NaCl, NaBr, KF, KCl, and KBr. Through quantitative polarization analysis of the SHG data, the orientational parameter $D$ $(D=⟨cosθ⟩/⟨cos3θ⟩)$ value and the relative surface density of the interfacial water molecules at these aqueous solution surfaces were determined. From these results, we found that addition of each of the six salts caused an increase in the thickness of the interfacial water layer at the surfaces to a certain extent. Noticeably, both the cations and the anions contributed to the changes, and the abilities to increase the thickness of the interfacial water layer were in the following order: $KBr>NaBr>KCl>NaCl∼NaF>KF$⁠. Since these changes cannot be factorized into individual anion and cation contributions, there are possible ion pairing or association effects, especially for the NaF case. We also found that the orientational parameter $D$ values of the interfacial water molecules changed to opposite directions for the aqueous solutions of the three sodium salts versus the aqueous solutions of the three potassium salts. These findings clearly indicated unexpected specific $Na+$ and $K+$ cation effects at the aqueous solution surface. These effects were not anticipated from the recent molecular dynamics simulation results, which concluded that the $Na+$ and $K+$ cations can be treated as small nonpolarizable hard ions and they are repelled from the aqueous interfaces. These results suggest that the electrolyte aqueous solution surfaces are more complex than the currently prevalent theoretical and experimental understandings.